The present invention relates to Radio Data System (RDS) signals broadcast with VHF radio transmissions, and in particular, to a method and circuitry for demodulating the RDS signal.
The Radio Data System, or RDS, is a information broadcast that was introduced for use in FM radio stations which transmit stereo-multiplex signals in the VHF frequency band. The Radio Data System provides radio receivers with broadcast data about the transmitting radio station and the programs broadcast by the radio station. The radio receivers typically reproduce this data on an optical display such as a liquid-crystal display screen.
The RDS broadcast data may include: program identification (PI) which indicates the program being received or the name of the station/transmitter tuned in; program type identification (PTY) which indicates the type of program such as music, news, etc.; traffic announcements (TA); and/or radio text (RT) which contains program-accompanying information such as the music title, performer, program changes, and the like.
The Radio Data System is used principally in car radios. For example, when the reception of the transmitter currently tuned in deteriorates, RDS-capable car radios automatically switch over to a better, or best receivable, transmitter broadcasting the same program. The information required to do this is included in the noted program identification (PI) information along with a list of alternative frequencies (AF) which are being broadcast by RDS-capable radio stations. The Radio Data System also offers advantages to the listener of home FM receivers as well. For example, the home listener can benefit from the noted program type identification (PTY) and radio text (RT) information.
As noted, the VHF signal transmitted by FM radio stations is referred to as a stereo-multiplex signal. A stereo-multiplex signal includes the following components: an audio center (mono) signal at up to 15 kHz; a stereo pilot tone at 19 kHz; a stereo signal between 23 kHz and 53 kHz, and an ARI (Autofahrer-Rundfunk-Information-System (German), referred to as Motorist Information System in the United States) signal.
The ARI signal component is a narrowband amplitude-modulated signal with a carrier frequency of 57 kHz. The RDS signal is a binary signal that includes of a continuous binary data stream with a bit rate of 1.1875 Kbits/s. The RDS signal, which has a greater bandwidth than the ARI signal, is superimposed on the ARI signal. The RDS signal is generated from the RDS data stream by double-sideband modulation with carrier suppression. In addition, the suppressed RDS carrier is phase-shifted by 90xc2x0 relative to the ARI carrier at 57 kHz. Because of this quadrature modulation, interference with the ARI signal by the RDS signal is essentially suppressed. In an RDS-capable radio transmitter, the carrier is frequency-modulated by the stereo-multiplex signal formed in the above-described manner and broadcast. To prevent the RDS signal from interfering with the other component signals such as the audio center signal, the stereo signal and the stereo pilot tone, while achieving a high data rate, the frequency spectrum of the RDS signal is typically restricted to xc2x12.4 kHz.
On the receiver side, the received frequency-modulated carrier is demodulated to obtain the stereo-multiplex signal from which the RDS signal as well as the audio signals are obtained.
One challenge for mobile FM receivers such as those installed in automobiles is that it can take a considerable amount of time before the tuner is synchronized to the 57 kHz carrier of the RDS signal due to constantly changing and often unsatisfactory reception conditions. Therefore, there is a need for a technique for demodulating the RDS signal such that fast synchronization with the carrier of the RDS signal is achieved.
Briefly, according to an aspect of the invention, a phase-locked loop circuit for demodulating a Radio Data System (RDS) signal superimposed on an ARI signal component of a stereo-multiplex signal is disclosed. The circuit comprises an oscillator that generates an in-phase component signal and a quadrature component signal of the carrier of the RDS signal, a first circuit branch comprising a first multiplier having a first input at which a sampled stereo-multiplex signal is received and a second input at which the in-phase component signal is received, a first low-pass filter having an input connected to an output of the first multiplier, a first divider having an input connected to an output of the first low-pass filter, and a first high-pass filter having an input connected to an output of the first divider; a second circuit branch comprising a second multiplier having a first input at which the sampled stereo-multiplex signal is received, and a second input at which the quadrature component signal is received, a second low-pass filter having an input connected to an output of the second multiplier, a second divider having an input connected to an output of the second low-pass filter, and a second high-pass filter having an input connected to an output of the second divider; a feedback branch comprising an arithmetic unit having first and second inputs connected to outputs of the first and second high-pass filters, respectively, a clock input at which an RDS bit clock signal is received, and an output at which the arithmetic unit generates an error signal; a filter having an input at which it receives the error signal, and an output at which it generates a filtered error signal; a control unit having an input connected to the filter output, and an output connected to a control input of the oscillator at which the control unit generates a control signal in response to the filtered error signal; a clock generator having a first control input connected to the output of the first high-pass filter, a second control input connected to an output of the oscillator, and an output at which the clock generator generates the RDS bit clock signal; and an RDS decoder having a first input connected to the output of the first high-pass filter, a clock input at which the RDS bit clock signal is received, and an output from which RDS data is retrievable.
Another aspect of the invention includes a method for demodulating a Radio Data System (RDS) signal superimposed on an ARI signal component of a stereo-multiplex signal. The method comprises generating an in-phase component signal and a quadrature component signal of the carrier of the RDS signal; multiplying a sampled stereo-multiplex signal by the in-phase component to generate a first product signal; low-pass filtering the first product signal to generate a first low-pass filtered signal; dividing a sampling rate of the first low-pass-filtered signal by a first presettable division factor to generate a decimated. filtered first product signal; high-pass filtering the decimated, filtered first product signal to generate a first high-pass-filtered signal; decoding the first high-pass filtered signal to generate RDS data; multiplying the sampled stereo-multiplex signal by the quadrature component to generate a second product signal; low-pass filtering the second product signal to generate a second low-pass filtered signal; dividing a sampling rate of the second low-pass-filtered signal by a second presettable division factor to generate a decimated, filtered second product signal; high-pass filtering the decimated, filtered second product signal to generate a second high-pass-filtered signal; calculating an error signal representing a phase difference between the carrier of the RDS signal and the output signal of an oscillator based on the first and second high-pass-filtered signals and an RDS clock signal, wherein the error signal represents a phase position between the carrier of the RDS signal and the output signal of the oscillator; and generating a correction signal for controlling the oscillator based on the error signal.
In a still further aspect of the invention, a phase-locked loop circuit for demodulating a Radio Data System (RDS) signal superimposed on an ARI signal component of a stereo-multiplex signal, the circuit comprising means for generating an in-phase component signal and a quadrature component signal of an RDS carrier signal in response to an oscillator control signal; means for generating a first product signal of a sampled stereo-multiplex signal and the in-phase component signal; means for generating a second product signal of a sampled stereo-multiplex signal and the quadrature component signal; means for controlling the oscillator based on the phase relationship between the RDS carrier signal and the signals generated by the oscillator; and means for generating RDS data based on the first high-pass filter.